The invention relates to a closed heating circuit useful for bus vehicles wherein a liquid heating medium in a liquid state is heated and passes through passenger heaters in a liquid vapor state such that the heat exchange takes place isothermally.
A passenger compartment heating system for a bus heating system with a closed circuit and heaters arranged in series is known in automotive heating technology (DE-OS 30 41 710). In order to reduce the energy requirement of heating systems of this kind, DE-OS 30 02 155 proposes the attachment of heat exchangers to units generating waste heat and devices which feed this waste heat to the heating circuit. In general however, a disadvantage of all serial heater arrangements of passenger compartment heating systems and, in particular, of bus heating systems is that due to the emission of heat, the heat transfer medium cools continuously. As a result there is a different temperature in each heater. In extreme cases this can lead to the heat at different heaters and consequently at different passenger compartment areas being subject to widely different temperatures.
It is the object of the invention to further develop the passenger compartment heating system such that the temperature is virtually the same in all heat-emitting heaters. This object is achieved by having the inlet temperature and the heat transfer medium matched to one another in such a way that a two-phase mixture of the liquid and vapor phase of the heat transfer medium in a saturation vapor state is present in the heat-emitting part of the closed circuit. Here the heat absorbing exchanger operates as a vaporizer(s), is designed essentially as a porous wall or as a corresponding capillary block, and is situated so as to separate the liquid side and vapor side of the closed circuit heating system.
The porous wall or the capillary block has capillary-active channels within the closed circuit and the heat supply is fed liquid through the porous wall or the capillary block to its vapor side.
A liquid/vapor mixture of a heat transfer medium is passed through the heaters. The medium is virtually completely vaporized on the vapor side of the vaporizer. As long as the heat transfer medium is in the form of a two-phase mixture (liquid/vapor mixture), the serially arranged heaters of the heat-emitting part have virtually the same temperature at saturated vapor temperature. This is to be attributed to the change in the state of aggregation of individual volumes of the heat transfer medium. The heat radiated by the heaters is produced by the latent heat stored in the vapor which is released in a virtually isothermal process upon condensation of the vapor.
It is advantageous if the capillary-active channels have an inside diameter in the range between 0.5 and 140 mm and if the shape of at least one surface region of the porous wall or of the capillary block is matched to the corresponding part of the surface of the heat generating unit. Also a part of the heat generating unit which touches the vapor-side surface of the porous wall or of the caPillary block (at least in certain areas) is desirably manufactured from thermally conductive material. At its contact region with the vapor-side surface it defines a surface-covering open channel structure to carry away the saturated vapor phase. These individual channels are closely spaced and the cross-section of the intermediate walls between two adjoining channels end like a knife edge pointing toward the porous wall or the capillary block. In certain regions, the surface-covering channel structure has a waffle-shaped, channel-shaped or knob-shaped structure.
It is also advantageous if a buffer tank of alterable or variable volume is provided for controlled removal or refilling of the liquid heat transfer medium. The tank should be connected in the region of the liquid side of the passenger compartment heating system. Also a collecting container for the heat transfer medium condensed in the region of the vapor side should be arranged geodetically on virtually the same level as the vaporizer. The collector is connected in the region of the vapor side, between vaporizer and the first heater of the heat-emitting part of the passenger compartment heating system. A valve controlled return line leads from the collecting container to the liquid side of the passenger compartment heating system.
It is also advantageous if a controllable pump is provided in the closed circuit in the region of the liquid side of the passenger compartment heating system.
To insure proper drainage, lines connecting the heat-emitting elements of the heat-emitting part of the closed circuit should always have a clear downward gradient towards the liquid side of the passenger compartment heating system.
To insure isothermal heating, at the pressure prevailing on the vapor side of the passenger compartment heating system, the boiling temperature of the heat transfer medium must be equal to or less than the temperature of the heat-emitting unit. Ideally the heat transfer medium is a fluorocarbon.
For ease in construction, the porous wall or the capillary block can consist of heat-resistant plastic or at least contain one of polytetrafluoroethylene or polyethylene or polystyrene.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.